1. Technical Field
The present invention relates to an image sensor and a method of its manufacture, and more particularly, to an image sensor having a 3-dimensional transfer transistor and a method of its manufacture.
2. Discussion of the Related Art
An image sensor is a semiconductor device that converts an optical image into an electric signal. The image sensor can be classified into either a charge coupled device (CCD) type or a complementary metal oxide semiconductor (CMOS) type. The CMOS type image sensor will be referred to as CIS (CMOS image sensor). The CIS includes a plurality of 2-dimensional pixels. The pixels each employ a photodiode (PD), a floating diffusion region (FD), and a transfer transistor (TX). The photodiode PD serves to convert incident light into an electric signal. The transfer transistor TX serves to transfer photocharges collected at the photodiode PD to the floating diffusion region FD. If the photocharges collected at the photodiode PD are not completely transferred to the floating diffusion region FD, there may occur a problem of a degraded display resolution by a phenomenon called an image lag. Thus, it is important that the transfer transistor TX sufficiently transfer the photocharges collected at the photodiode PD to the floating diffusion region FD.
A method of manufacturing such an image sensor has been disclosed in U.S. Patent Publication No. 2003/0173585 A1, Kimura, et al., “Semiconductor device having solid-state image sensor with suppressed variation in impurity concentration distribution within semiconductor substrate and method of manufacturing the same.”
FIG. 1 is a cross-sectional view of a pixel portion of the image sensor disclosed in U.S. Patent Publication No. 2003/0173585 A1.
Referring to FIG. 1, a P-well 11 is formed on the semiconductor substrate 10 at a certain depth. An isolation layer 12 is formed to define an active region. A transfer gate structure 15 is disposed across on the active region. The transfer gate structure 15 includes a gate dielectric 13 and a transfer gate electrode 14 on top. A photodiode 18 is disposed in the P well 11 at one side of the transfer gate structure 15. The photodiode 18 has a P-type impurity region 16 and an N-type impurity region 17. A floating diffusion region 19 is disposed in the P well 11 at the other side of the transfer gate structure 15.
For a high integration of the image sensor, the sizes of the photodiode 18 and the transfer gate structure 15 should be reduced as much as possible. However, a drivability of the transfer transistor TX is determined by the length and width of its effective channel. The width of the effective channel is defined by the transfer gate structure 15. That is, for securing the drivability, it is advantageous to enlarge the width of the effective channel defined by the transfer gate structure 15. However, in the publication by Kimura, et al., the transfer gate structure 15 is disposed on the plane on the semiconductor substrate 10. Accordingly, there is a trade-off between the drivability and the reduction of the size of the transfer structure 15.
Using a 3-dimensinal structure transfer transistor may be a solution of the above problem, as we now discuss.
FIG. 2 is a plan view illustrating a part of a pixel portion of an image sensor having a conventional 3-dimensional (3-D) transfer transistor, and FIG. 3 is a cross-sectional view taken along line I-I′ of FIG. 2.
Referring to FIGS. 2 and 3, the image sensor having the conventional 3-D transfer transistor includes, at a certain area of a semiconductor substrate 21, an isolation layer 23 defining a first active region 1 and a second active region 2. The second active region 2 is configured to extend from the first active region 1. A shallow p-impurity region 27 is in a portion of the first active region 1. A deep n-impurity region 25 is below the shallow p-impurity region 27. The shallow p-impurity region 27 and the deep n-impurity region 25 define a photodiode (PD) 29. A floating diffusion region (FD) 39 is in the second active region 2 at a position spaced apart from the photodiode 29. The floating diffusion region 39 is a high concentration n-impurity region. A transfer gate electrode 33 is disposed on the second active region 2 between the photodiode 29 and the floating diffusion region 39. The transfer gate electrode 33 is so disposed across the second active region 2 as to cover the sidewalls 37 of the second active region 2.
The transfer gate electrode 33 has a region that extends onto the first active region 1 and overlap the photodiode 29. In this case, a spaced region d is formed between the sidewalls 37 of the second active region 2 and the photodiode 29. Also, a gate dielectric 31 is interposed between the transfer gate electrode 33 and the active regions 1 and 2. The gate dielectric 31 may further extend to cover the photodiode 29 and the floating diffusion region 39. The deep n-impurity region 25, the transfer gate electrode 33 and the floating diffusion region 39 constitute a transfer transistor TX. In this case, the width of an effective channel of the transfer transistor TX is determined by the upper portion 35 and sidewalls 37 of the second active region 2. That is, the width of the effective channel is enlarged to the extent of a length of the sidewalls 37 of the second active region 2 covered by the transfer gate electrode 33.
If incident light is radiated onto the photodiode 29, the photocharges are concentrated in the deep n-impurity region 25. The quantity of the photocharges concentrated in the deep n-impurity region 25 is determined by the intensity of incident light. Subsequently, when a gate voltage higher than the threshold voltage is applied to the transfer gate electrode 33, an inversion region is formed at the upper portion 35 and sidewalls 37 in the active regions 1 and 2 covered with the transfer gate electrode 33, and the photocharges concentrated in the deep n-impurity region 25 are transferred to the floating diffusion region 39 via the inversion region.
However, if the spaced region d exists, the photocharges concentrated in the deep n-impurity region 25 have difficulty moving toward the sidewalls 37 of the second active region 2. That is, the transfer efficiency of the photocharges is degraded.